The present invention relates to a technique for packing a permanent magnet powder uniformly in a cylindrical molding space provided in a mold.
This technique can be utilized for molding of rare earth, ferrite and metallic magnets, production of a bonded magnet, etc. A method which comprises packing a permanent magnet powder into a molding space of a mold and subjecting the packed powder to compression molding with a pressing machine has been generally adopted for molding of a permanent magnet pole for the purpose of developing excellent magnetic charcteristics.
In conventional compression molding, the packing of the permanent magnet powder into the molding space of the mold has been exclusively conducted by taking advantage of gravity drop. Specifically, in a general method of packing powder, a predetermined amount of a powder is supplied into a molding space of a die by leveling in such a manner that a powder is dropped by gravity through the opening of a molding space by a bottomless powder feeding mechanism sliding on the top surface of a die which constitutes a mold and the powder feeding mechanism is then slid to remove excess powder.
When the area of the opening of the molding space of the mold is considerably large, it is possible to pack the powder by the above-described method. However, for example, in the case of a thin-wall cylindrical molding space having a small opening width, it is difficult to conduct sufficient packing through mere sliding of the powder feed mechanism along the top surface of the mold.
For this reason, other methods are sometimes adopted such as one wherein packing is conducted by vibrating the mold and one wherein a blade for forced packing (forcing member) is provided on the upper part of the molding space to force the powder into the molding space.
The method wherein packing is conducted by vibrating the mold has drawbacks such as high susceptibility to variation in the packing rate depending upon the state and shape of powder, risk of fastening of an apparatus with a bolt etc. to be loosened due to the vibration, poor improvement in the packing efficiency, and poor reproducibility of the amount of packing.
The method wherein forced packing is conducted with a blade has drawbacks such as very poor production efficiency due to not only a necessity of long period of time for packing but also difficulty of automating the packing of powder in the step of pressing, localized ocurrence of a difference in the shape between the packed powder and the powder before packing due to forcing of the powder by the blade which makes it difficult to conduct uniform packing of a predetermined amount of powder and therefore brings about a variation in the weight of the packed powder and non-uniform density distribution during molding.